Coal against biomass, measured at the lungs rather than the ledger

Most plants compare fuels by calorific value per ton. This one compares them by what turns up in workers' lungs and in the water around the site: the cost that never appears on a quotation.

What leaves the stack when coal burns

PM2.5: particulate fine enough to pass through the alveoli into the bloodstream, linked to respiratory and cardiac disease.

Sulphur dioxide. Coal carries significant sulphur; burning it produces SO2, the cause of acid rain and acid corrosion in the stack.

Nitrogen oxides, formed at high combustion temperature, which react in the atmosphere to form secondary particulate.

Heavy metals: mercury, arsenic and cadmium carried in the coal, accumulating in soil and water through ash and leachate.

How biomass differs

Far less sulphur. Wood carries tens of times less sulphur than coal, so SO2 falls substantially and acid corrosion in the stack with it.

Carbon on a short cycle. A tree absorbs CO2 while growing and releases it when burned, on a cycle of roughly 25–35 years for a rubber tree, against coal releasing carbon locked underground since long before it.

The part that has to be said plainly

Biomass is not pollution free. Combustion still emits particulate and NOx, and burned incompletely it emits more than it should.

Wood ash is high in potassium, which lowers the ash fusion temperature and means furnace temperature has to be controlled to avoid slagging. That is a technical problem to manage, not an advantage.

Wood contains trace heavy metals, not none. Arsenic, cadmium, mercury and lead are present in small amounts, which is why the ENplus pellet standard caps them (arsenic ≤1, cadmium ≤0.5, mercury ≤0.1 mg/kg).

The genuine advantages are much lower sulphur and the short carbon cycle. Not zero emissions. Anyone telling you biomass is clean is selling something.

The part usually left out: field burning

In Southern Thailand, roots, branches and leaves left after replanting are commonly burned in the open, with no air control, no dust collection and many fields at once in the dry season.

Bringing that material into a controlled furnace therefore cuts particulate by more than a fuel-for-fuel comparison suggests, because it displaces the worst burning at the source.

Read on: rubberwood stumps and roots

If you are weighing a change

Start by co-firing at a low ratio and collect real data. See the method on the replacing coal with biomass page, or call +66 92 392 2292.

Sources

• WHO, Types of pollutants
• US EPA, What is Acid Rain?
• US EPA, Basic Information about NO2
• US EPA, AP-42 §1.1 Bituminous and Subbituminous Coal Combustion
• US EPA, AP-42 §1.6 Wood Residue Combustion in Boilers
• ENplus, ST 1001 Requirements for companies
• ERIA, Chapter 2 rubberwood supply
• Frontiers in Energy Research, Ash and potassium in biomass (2025)
• กรมส่งเสริมการเกษตร, ข่าว 29 มี.ค. 2567 (29 มี.ค. 2567)